JSON Schema requires two fields to include the $ prefix.
First is the $schema keyword that defines the version and URI of the schema to be stored.
Second is the $ref used to describe pointers to additional schema data.
More info on JSON Schema is available here: http://json-schema.org/documentation.html
As JSON Schema is becoming a more heavily used tool for JSON data management
I see it as a useful exception to the no $ prefix error and it could even be useful to allow $ref resolution and caching in future (as a separate issue).